Conventionally, crosslinked carboxyl group-containing polymers are widely used in, for example, thickeners for cosmetics and the like, moisturizers for cataplasms and the like, emulsifiers, suspension stabilizers for suspended matter and the like, or gel bases for batteries and the like. Examples of those known as such crosslinked carboxyl group-containing polymers include a copolymer of an α,β-unsaturated carboxylic acid such as acrylic acid and a polyallyl ether (Patent Literature 1), a copolymer of an α,β-unsaturated carboxylic acid and hexaallyl trimethylene trisulfone (Patent Literature 2), a copolymer of an α,β-unsaturated carboxylic acid and triallyl phosphate (Patent Literature 3), and a copolymer of an α,β-unsaturated carboxylic acid and glycidyl methacrylate (Patent Literature 4).
These crosslinked carboxyl group-containing polymers, in the form of a neutral viscous solution prepared by dissolution in water, followed by neutralization with a neutralizer such as an alkaline compound, are used for purposes as described above.
A crosslinked carboxyl group-containing polymer needs to be prepared as a homogeneous aqueous solution, in order to be used for these purposes. A crosslinked carboxyl group-containing polymer, however, is generally produced as a powder, and when the powdery crosslinked carboxyl group-containing polymer is dissolved in water, masses (lumps) tend to be formed. Once lumps are formed, a gel layer is formed on their surface, slowing down the rate of penetration of water into the inside, making it difficult to obtain a homogeneous solution. Thus, when a crosslinked carboxyl group-containing polymer is used, in order to prevent the formation of lumps, it is necessary to gradually add the crosslinked carboxyl group-containing polymer into water with stirring at high speed, which is an operation with poor production efficiency, and even a special dissolution apparatus is sometimes required to prevent the formation of lumps.
On the other hand, a neutral viscous solution having a higher viscosity can find a wider range of uses as a thickener, and can also achieve a reduction in the amount of use. In recent years, therefore, the development of a polymer for use as a thickener that imparts a high viscosity has been eagerly anticipated.
To provide a crosslinked carboxyl group-containing polymer in which the formation of lumps as described above has been inhibited, Patent Literature 5, for example, proposes a technique that involves adding a specific amount of at least one compound of a polyhydric alcohol fatty acid ester and a polyhydric alcohol fatty acid ester alkylene oxide adduct when preparing a carboxyl group-containing polymer by polymerization of an α,β-unsaturated carboxylic acid and the like.